Substituted 2,3-pyridinedicarboxylic acids are important intermediates in the manufacture of highly effective 2-(2-imidazolin-2-yl)nicotinate herbicides. Among the methods to prepare substituted 2,3-pyridinedicarboxylic acids is the nitric acid oxidation of the appropriately substituted quinoline precursor. However, certain substituted pyridinedicarboxylic acids are difficult to isolate from the spent nitric acid solution. Such compounds do not readily precipitate from the product solution.
The compound, 2,3-dicarboxypyridinium nitrate is described by P. Sutter and C. Weis in the Journal of Heterocyclic Chemistry, 23 p. 29-32 (1986), however no substituted 2,3-dicarboxypyridinium nitrates are found therein.
Therefore, it is an object of this invention to provide crystalline substituted 2,3-dicarboxypyridinium nitrate compounds which are useful in the isolation and purification of important 2,3-pyridinedicarboxylic acid herbicide intermediates. It is another object of this invention to provide a means for producing the desired substituted pyridinedicarboxylic acid herbicide intermediates in improved yield and purity.
It is a feature of this invention that the desired dicarboxylic acid product may be obtained without necessitating a quenching step, thereby allowing the spent nitric acid to be recycled and eliminating the costly and potentially hazardous presence of solvent wastes.